Electronic structure in the band gap of lightly doped SrTiO3 by high-resolution x-ray absorption spectroscopy

The electronic structure of lightly Nb-doped SrTiO3 has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O 1s threshold, XAS spectra show two features due to empty stales whose energy positions match with those of the 3d photoemission spectra in the band gap energy region of the parent undoped SrTiO3. Similar features are also observed in lightly La-doped SrTiO3. These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states.